Ok, after reading everyones suggestions i suggest maybe 2 game modes if people can do the effort with a co-op type and normal pvp type. Insurgency checkpoints and caches, deployables and more limited where you can deploy maybe 10 wires at an one time and they can be destroyed by zombies (cuz you know they dont feel pain...), if you want vehciles maybe a disabled apc at random spots (if its not hardcoded) works a bit maybe like a deployable 50 except you dont build it(make sense?), scatterd human spawns, a lot of blood...

, if you can do a civi kit with guns that be good.
Objectives: Survive,a to b, maybe a bit of vanilla conquest where you have to kill a certain amount of humans-zombies (for pvp), a capture the flag sort of thing except you carry idk fuel (lfd2 inspired)
Zombies: For AI limit them to walk yet only maybe a bit raised health?, blood again (just dont over do it), if your going to do classes may i suggest you give the zombies the rope for city maps so they can crawl all over yet they take a bit more damage from falls giving their fraigle (atleast some of them) bodies.
Survivers: Keep most of the kits removing maybe HATs except for the rpg but from caches, a leader class( officer) who can deploy assets, nice death scenes (though thats just a fantasy with a bf2 engine), almost a taliban kit with molotovs and stuff with M9s or any pistols from the civi approach, for normal armies you should lower ammo amounts because the armed forces got more guns than a normal person yet they cant get a constant stream of supplies in because of all the zombies.
Maps: Muttrah and Fallujah with dynamic lighting if you developers are still working on that would be epic, for big open forest maps it maybe a bit harder cant think of anything for those, i dont like the idea of desert maps but if its like part of the enviroment (Al Basrah) the scaterd spawn will work good with this.
And most importantly, zombie sounds and good music, blood (sorry just had to put that in there) could set the mood just nicely. WHEW